1. Field of the Invention
The present invention relates to a target substance-detecting apparatus for detecting a target substance by using a plasmon resonance phenomenon of a metal, and a target substance-detecting method therefor.
2. Description of the Related Art
There are a plurality of disease markers for particular diseases such as malignant tumor, hepatitis, diabetes mellitus and osteoporosis, in blood, and when a person is afflicted with a disease, the concentration of a particular protein changes (increase or decrease) compared to a usual condition. For this reason, it leads to an early detection of serious diseases to monitor the disease markers in normal times, which is expected as the next generation of a medical technology.
In addition, it is thought that the transition and recurrence of a tumor can be early detected by monitoring a tumor marker relating to the malignant tumor after the tumor has been removed, and the monitoring is expected to largely contribute to the improvement of medical quality.
In recent years, Food Sanitation Law has been revised, and business undertakers are required to display egg, peanut, buckwheat, wheat and milk on their products when they are contained in a food. However, there are few people who know, quantitatively, how much allergic predisposition they have and to which kind of foods. Accordingly, it is important to scientifically know these data as a numeric value, from the viewpoint of health promotion and preventive medical practice for each person.
A method of analyzing a raw and unpurified protein occasionally employs a sensor which identifies a particular compound by using a biological ligand-analyte interaction.
There are several types of sensors which employ several detection methods such as a fluorescent immunoassay technique, a plasmon resonance technique and a light interference technique. Any type of the techniques commonly has the steps of immobilizing a ligand on the surface of the sensor, and making the ligand bonded to an analyte in a specimen with high sensibility and high selectivity to remove concomitants and to catch only an objective analyte (target substance) with high efficiency.
The plasmon resonance technique measures the concentration of the target substance specifically bonded to the ligand immobilized on a metallic thin film or fine particles of a metal, by using a phenomenon that a metal responds to a change of an optical refractive index of an interface material with high sensibility. The above ligand is immobilized on the surface of the sensor by using a chemical or a physical technique.
The plasmon resonance technique can obtain information on the change (kinetics) with time of a reaction between the ligand and the analyte, which is thought to be an advantage in addition to a feature of needing no labeling.
As for an apparatus using the plasmon resonance, various techniques are proposed in order to improve a detection performance for the target substance. The techniques are disclosed, for instance, in Japanese Patent Application Laid-Open No. 2005-156414 and Japanese Patent Application Laid-Open No. 2001-513198.
An apparatus which is disclosed in Japanese Patent Application Laid-Open No. 2005-156414 is an apparatus which polarizes an incident light into a P wave and an S wave, then irradiates the sensor with the lights of the waves, makes a photodetector measure the intensity of reflected light based on a signal from the P wave, and sense the intensity of a light source or the change of sensibility of the photodetector based on a signal from the S wave. Thereby, the apparatus can accurately detect the target substance even when the intensity of the light source or the sensibility of the photodetector has changed.
An apparatus which is disclosed in Japanese Patent Application Laid-Open No. 2001-513198 is an apparatus which intends to improve its detection sensibility for the target substance, and which detects light scattered by metal microparticles with the photodetector while using a dark field optical system suitable for detecting the microparticles, and analyzes the detection contents of the photodetector with an image processor to specify the target substance.
When a target substance is detected by using plasmon resonance, and when metal structure have a rectangular shape, for instance, and have anisotropy in a plane perpendicular to an incident light, the target substance-detecting element greatly varies its optical characteristics depending on a direction in which the target substance-detecting element has been irradiated with the incident light. Thereby, a plasmon resonant state does not significantly appear, and the apparatus may lower its detection accuracy for the target substance.
Even the apparatuses disclosed in Japanese Patent Application Laid-Open No. 2005-156414 and Japanese Patent Application Laid-Open No. 2001-513198 are not provided with a device considering the shape of the target substance-detecting element, when the target substance-detecting element is irradiated with an incident light, and accordingly are thought to occasionally cause the above described problem.
The present invention has been designed so as to solve the above described problem, and is directed at providing a target substance-detecting apparatus for detecting a target substance with high accuracy and a target substance-detecting method therefor.